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Four wires: black, red, blue and white

Black and red are for a 30a dryer.

Blue is for 120v/20a circuit. All #10 gauge. Can the neutral be used for both circuits?

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  • The only way that can possibly work is if the utility supply is 3-phase 120/208V. At that point you have a simple 3-phase MWBC with the dryer on 2 legs and the socket on the 3rd. However, the breaker would be impossible. Common dryers need 30A with common trip and the 120V outlet can't have more than a 20A breaker. They all need to be handle tied, so you can't sandwich a 230 and 120. Nobody makes a 30-30-20 3-pole breaker with common trip! Unless it feeds a 30A travel trailer TT30 socket, only possibility. TLDR NO. Commented Nov 28, 2024 at 0:31
  • On the other hand you can get a 240V-only dryer. That would be a heat pump dryer. OR, I've heard some dumb old resistance electric dryers are now being made in 240V-only. At that point black/red are the dryer circuit and blue/white are the 120V circuit. Commented Nov 28, 2024 at 0:36

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No, for circuit that have a neutral there must be one neutral for each breaker. you can share grounds, but you can't share neutrals.

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  • Why can grounds be shared? Because they hardly ever get a load, and in the event they do, it's only from a single circuit? Commented 2 days ago
  • grounds can be shared because they never carry current for long enough to start a fire, mismatched neutrals can cause all sorts of problems. Commented yesterday
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The way to think about it is to pretend that every circuit is protected a GFCI device, which isn't far from the truth in new construction. In order for GFCI to work, all hots + neutral in each circuit must be balanced. The 30A two hots and neutral are balanced. The 20A hot and its neutral are balanced. But if you combine the neutrals then there is no way to send just the right amount of current to each GFCI.

Note that this is per circuit. A circuit can be:

  • 120V - hot/neutral
  • 240V - hot/hot, no neutral (e.g., water heater)
  • 240V/120V - hot/hot/neutral (e.g., clothes dryer or MWBC).

An MWBC might be where you get this idea of "share neutral". But an MWBC doesn't share a neutral between two different circuits. It is really one circuit, despite the historical situation of using two separate breakers instead of a double breaker. Technically, an MWBC can still be two separate breakers, as long as they are next to each other and handle-tied. But a new MWBC in an area that requires GFCI (e.g., kitchen, bathroom, laundry room, garage) has to be a double breaker in order for the GFCI to work.

If these wires are in conduit then just add a 12 AWG neutral. If these wires are part of a cable then you need a separate cable for the 20A circuit, but that can be a 12 AWG /2 instead of a 10 AWG /4.

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